1. Field of the Invention
The present invention relates to a constant current circuit which is manufactured as a semiconductor integrated circuit, and in particular to achievement of stable characteristics relative to varying temperature.
2. Description of the Related Art
Conventionally, a variety of constant current circuits have been conceived with various ideas applied thereto to obtain a constant current which is less affected by the influence of varying temperature.
FIG. 1 is a diagram showing a structure of a conventional constant current circuit. Specifically, Metal Oxide Semiconductor Field Effect Transistors (MOSFET) Q1 and Q2, and MOSFETs Q3 and Q4, respectively constitute current mirror circuits, and these transistors Q1 through Q4 operate such that equivalent currents I flow through a first path which includes the MOSFETs Q1 and Q4 and a second path which includes the MOSFETs Q2 and Q3. The gate of the MOSFET Q5 is connected to the gate of the MOSFET Q4 which has a gate and a drain being short-circuited. As a result, the pair of the MOSFETs Q4 and Q5 also constitute a current mirror circuit in which a current which is equivalent to the currents I flowing through the first and second paths, respectively, is extracted from the drain of the MOSFET Q5 as an output of the constant current circuit.
Further, in the circuit shown in FIG. 1, as a structure to suppress the influence of varying temperature, a resistance element R1 and a PNP transistor Q6 are serially connected between the source of the MOSFET Q1 and the ground, while a PNP transistor Q7 is serially connected between the source of the MOSFET Q2 and the ground. The MOSFET Q6 is set n time the size of the MOSFET Q7. Each of the MOSFETs Q6 and Q7 is a diode-connected transistor in which the base and collector thereof are short-circuited. Based on the current-voltage characteristics of the respective MOSFETs Q6 and Q7 in the above described condition and with a condition that equivalent voltages are applied to the MOSFET Q7 and the serially connected MOSFET Q6 and resistance R1, respectively, the current I takes the value obtained by the following expression.I=VT·ln(n)/R1  (1)wherein VT represents a heat voltage which is expressed as follows using electron charge q, Boltzmann constant k, and absolute temperature T,VT=kT/q  (2)
A typical resistance element, such as a discrete resistance element, has positive temperature characteristics, and, as is obvious from Expression (2), a heat voltage VT also has positive temperature characteristics. Therefore, change of the current I due to varying temperature can be suppressed as the respective positive temperature characteristics of the heat voltage VT and the resistance R1 are mutually offset in the current I given by Expression (1).
Here, in CMOS (Complementary Metal Oxide Semiconductor) processing, for example, a PNP transistor can be prepared as a parasitic element which has a collector formed utilizing a P-type semiconductor substrate (P-sub). Therefore, it is possible to manufacture a constant current circuit as shown in FIG. 1 in the form of a semiconductor integrated circuit manufactured through CMOS processing.
However, in CMOS processing, a resistance element, such as a poly-silicon resistance or the like, having negative temperature characteristics may be formed. In such a case, a problem is expected in which the circuit shown in FIG. 1 cannot enjoy the effect that the temperature characteristics of the heat voltage VT and the resistance R1 are offset to each other. On the contrary, the heat voltage VT and the resistance R1 may synergistically work on such that the temperature characteristics of the current I is enhanced in the position direction.